This invention relates to a process for the preparation of poly(organophosphazenes). More particularly, the invention relates to a process for preparing poly(organophosphazenes) which involves reacting a poly(dichlorophosphazene) with an active hydrogen-containing compound in the presence of a bicyclic amidine (described below).
Poly(organophosphazenes) to which the process of this invention is directed are polymers characterized by repeating units of the structure: ##STR3## in which X and X' represents organic substituents which may be the same or different groups derived from active hydrogen-containing compounds and in which n is from 20 to 50,000. As will be evident, when X and X' in the above formula are the same, homopolymers are produced, whereas when X and X' are different, copolymers are produced.
The process of the invention is particularly advantageous in preparing poly(organophosphazenes) in which X and X' in the above formula represent substituted or unsubstituted alkoxy or aryloxy groups; i.e., polymers containing repeating units of the structure: ##STR4## in which R and R' may be the same or different and are substituted or unsubstituted alkyl or aryl radicals.
Conventional processes for the preparation of poly(organophosphazenes) from poly(dichlorophosphazenes) are described in numerous prior art patents and publications as illustrated, for example, by the publications "Phosphorus-Nitrogen Compounds" Academic Press, New York, New York 1972 by H. R. Allcock and "Poly(Organophosphazenes)" Chemtech, Sept. 19, 1975 by H. R. Allcock. As described in the foregoing publications, poly(organophosphazenes) are in general prepared from poly(dichlorophosphazene) polymers by a nucleophilic substitution reaction in which the chlorine atoms on the poly(dichlorophosphazene) polymer are displaced by the desired substituents. This reaction process is commonly referred to as derivatization by those active in the polyphosphazene polymer art.
As specifically described in the aforementioned publications, the general procedure for preparing poly(organophosphazenes) in which the organic substituents are alkoxy or aryloxy groups involves reacting a poly(dichlorophosphazene) with the sodium salt of an aliphatic or aromatic alcohol, i.e., a sodium alkoxide or aryloxide. The process described in the aforementioned publications is advantageous in many respects and permits the production of poly(organophosphazene) polymers containing various alkoxy and aryloxy substituents. However, the process described in these publications suffers from several serious disadvantages.
Thus, the first step in such processes involves the preparation of the sodium alkoxide or aryloxide of the desired aliphatic or aromatic alcohol. As described in the prior processes, this is ordinarily accomplished by reacting the aliphatic or aromatic alcohol with metallic sodium in an inert solvent.
As will be evident, the use of metallic sodium in the preparation of the alkoxide or aryloxide presents a number of significant disadvantages. Thus, as is well known, the use of metallic sodium requires extreme care in handling in order to prevent potential explosion problems. In this regard, it is necessary to rigorously exclude moisture from the reaction system.
Thus, the preparation of poly(organophosphazenes) in which the organic substituent is an alkoxy or aryloxy group in accordance with prior processes basically involves a two-step process, i.e., (1) prepare the dry alkoxide and (2) react the alkoxide with the poly(dichlorophosphazene) polymer. As will be apparent, such a two-step process requires not only careful handling but is time consuming.
The use of metallic sodium in such processes presents still further disadvantages. For example, the use of sodium has the additional disadvantage that undesirable side reactions often occur. Thus, in some instances, it may be desirable to attach a functional group-containing substituent (e.g., an amino acid, nitrophenol, hydroxy acid etc.) to the phosphazene backbone of the polymer to provide a site for subsequent reaction such as crosslinking, grafting and the like. However, the use of sodium in such a derivatization reaction is generally undesirable since side reactions between the sodium and the functional group can readily occur.
Other processes for preparing poly(organophosphazenes) of the above structure from poly(dichlorophosphazenes) which are less frequently employed involve reacting the poly(dichlorophosphazene) with the desired alcohol in the presence of a tertiary amine such as triethylamine or pyridine. Such processes are described, for example, in U.S. Pat. No. 3,893,980; Japanese Patent publication 73-32800, filed Nov. 28, 1970 and the article entitled "Phosphonitrilic Trifluoroethoxide High Polymer" by M. V. Lenton, B. Lewis and C. A. Pearce appearing in the publication Chemistry and Industry, dated Aug. 1, 1964.
The processes described in the aforementioned patents and publications (hereinafter referred to as tertiary amine processes for convenience) are advantageous in a number of respects in comparison to the sodium processes described above. Thus, the tertiary amine processes are one-step processes in which the handling problems associated with sodium are substantially obviated and in which side reactions are virtually eliminated or at least greatly minimized. However, while the tertiary amine processes exhibit such advantages in comparison to the sodium processes, they have also been found to suffer from several significant disadvantages.
Thus, it has been found that in some instances the tertiary amine process produces poly(organophosphazene) polymers having a lower than desired degree of substitution (i.e. polymers containing higher than desired residual chlorine levels). This is clearly indicated by Japanese Patent publication 73-32800 which discloses a process in which a first substituent (i.e. phenoxy) is attached to the phosphorus atoms of a poly(dichlorophosphazene) by the tertiary amine process following which a second substituent (i.e. an unsaturated oxybenzoate ester) is attached to the phosphorus atoms of the poly(dichlorophosphazene) by the sodium process. The stated reason for utilizing both processes is to obtain a more complete substitution (i.e. low residual chlorine level). In addition, as illustrated in the aforementioned article by M. V. Lenton et al, the use of the tertiary amine processes in some instances produces polymer products which are at least partially crosslinked. Finally, in the tertiary amine process, the tertiary amine hydrochloride salt is formed as a by-product of the reaction. As illustrated in U.S. Pat. No. 3,893,980 to Allcock et al, this salt is difficult to separate from the polymer and numerous purification steps are often required to isolate the polymer.